Field of the Disclosure
Aspects of the present disclosure generally relate to data storage systems, and more particularly, to a voltage-controlled magnetic anisotropy (VCMA) switching device using an external ferromagnetic biasing film.
Description of the Related Art
Modern electronic devices increasingly incorporate significant amounts of solid state memory. The electronics industry continually seeks for higher density devices that provide low power consumption. Magnetic memory devices by their nature provide non-volatile characteristics, and are drawing increasing attention as a next generation memory type.
Higher storage bit densities in magnetic media used in disk drives have reduced the size (volume) of magnetic bits. Magnetic random access memory (MRAM) offers fast access time, infinite read/write endurance, radiation hardness, and high storage density. Unlike conventional RAM chip technologies, MRAM data is not stored as electric charge, but is instead stored by magnetic polarization of storage elements. MRAM cells including magnetic tunnel junction (MTJ) memory elements can be designed for in-plane or perpendicular magnetization of the MTJ layer structure with respect to the film surface. The elements are formed from two magnetically polarized plates, each of which can maintain a magnetic polarization field, separated by a thin insulating layer, which together form a MTJ stack. FIG. 1 is a diagram illustrating an example MTJ stack 100, according to certain aspects of the present disclosure. As shown in FIG. 1, one of the two plates is a permanent magnet 102 (i.e., has fixed magnetization) set to a particular polarity; the polarization of the other plate 106 will change (i.e., has free magnetization) to match that of a sufficiently strong external field. Therefore, the cells have two stable states that allow the cells to serve as non-volatile memory elements.
A memory device may be built from a grid of such cells. The MRAM cells in an array on a chip are connected by metal word and bit lines. Each memory cell is connected to a word line and a bit line. The word lines connect rows of cells, and bit lines connect columns of cells. Typically complementary metal-oxide semiconductor (CMOS) structures include a selection transistor which is electrically connected to the MTJ stack through the top or bottom metal contacts. The direction of the current flow is between top or bottom metal electrodes.
Reading the polarization state of an MRAM cell is accomplished by measuring the electrical resistance of the cell's MTJ. A particular cell is conventionally selected by powering an associated transistor that switches current from a supply line through the MTJ layer to a ground. Due to the tunneling magnetoresistance effect, where quantum tunneling of electrons through the tunneling barrier layer 104 occurs, the electrical resistance of the cell changes due to the relative orientation of the polarizations in the two magnetic layers of the MTJ. By measuring the resulting current, the resistance inside any particular cell can be determined, and from this the polarity of the free writable (free) layer determined. If the two layers have the same polarization, this is considered to mean State “0”, and the resistance is “low,” While if the two layers are of opposite polarization the resistance will be higher and this means State “1”. Data is written to the cells using a variety of techniques.
In conventional MRAM, an external magnetic field is provided by current in a wire in proximity to the cell, which is strong enough to align the free layer. Spin-transfer-torque (STT) MRAM uses spin-aligned (“polarized”) electrons to directly torque the domains of the free layer. Such polarized electrons flowing into the free layer exert a sufficient torque to realign (e.g., reverse) the magnetization of the free layer.
Magnetoresistive RAM (MeRAM) uses the tunneling magnetoresistance (TMR) effect for readout in a two-terminal memory element, similar to other types of MRAM. However, the writing of information is performed by VCMA at the interface of the tunnel barrier and the free layer, as opposed to current-controlled (e.g. STT or spin-orbit torque, SOT) mechanisms. In VCMA devices, magnetic properties are controlled by the application of an electric field. VCMA devices are based on electric-field-induced switching of nanomagnets. MeRAM devices have the potential for dramatic reductions in power dissipation. By eliminating the need for currents to operate the device, Ohmic dissipation is significantly reduced, resulting in a very low dynamic (i.e. switching) energy dissipation. In addition to reduced power dissipation, the use of electric fields for writing in MeRAM offers an advantage in terms of enhanced bit density. In particular, magnetoelectric writing does not impose a current-drive-based size limit on the access devices (e.g. transistors) when integrated in a circuit, hence allowing for much smaller overall cell area. At the same time, MeRAM in principle retains all key advantages of STT-MRAM, namely high endurance, high speed, radiation hardness, and possibility for nonvolatile operation.
Accordingly, a need exists for high density and high energy efficient magnetic memory devices.